A gap dimensions design method of expansion ring seal based on theoretical calculation and CFD simulation was proposed.The flow field numerical model of expansion ring seal was established based on fundamental equations of fluid dynamics.The confinement ranges of the radial clearance and axial clearance between the expansion ring seal ring and the shaft groove,and the clearance between the valve bushing and the rotating shaft were determined according to actual working conditions and application requirements.The computational fluid dynamics model for flow field was built and high sensitivity factors affecting sealing performance were found out by orthogonal experimental design,and best gap dimensions were chosen targeting minimum leakage.The results show that compared with the radial clearance and axial clearance between the expansion ring seal ring and the shaft groove,the clearance between the valve bushing and the rotating shaft has more significant influence on the leakage of the seal ring.With the increase of the clearance between the valve bushing and the rotating shaft,the leakage is also increased.